There have always been a number of problems associated with the use of adhesive-backed papers such as those used as labels, etc. These problems, largely related to avoiding premature contact of the adhesive with other objects, have been largely overcome by the use of releasable backing sheets. The need for convenient removal of labels or other adhesive papers from these release-coated backing sheets raised new problems and a considerable amount of inventive effort has been undertaken in solving such problems.
For example, U.S. Pat. No. 3,035,957 to Morgan disclosed a "chemical embrittlement" process which is used to provide embrittled lines along a backing sheet and thereby facilitate the breaking of the backing sheet and the mechanical removal of the sheet from the adhesive. Such a process avoids the need to mechanically score and cut through the release-coated sheet. The avoidance of cut-through eliminates the problem associated with the seepage of adhesive through the cuts. On the other hand, use of chemical compositions present handling problems in the plant, leave an unsightly marking on the sheet, and have a tendency to result in a product that will flake and lint in use.
U.S. Pat. No. 2,319,272 to Starr includes a disclosure of other methods such as knife-scoring, scorching and abrasion of a sheet, to selectively weaken linear portions therealong, on an adhesive-coated rubber patch. Starr's rubber patch may provide an especially high degree of cushioning for the backing sheet being processed; the Starr patent does teach the utility of such support, however.
Thus it is seen that the prior art has utilized various linear weakening techniques such as applying thin lines of acid to chemically weaken selected linear portions of the backing sheet, abrading the sheet to selectively weaken it along the abraded line, cutting through part of the sheet along predetermined lines, etc.
One problem which has interfered with obtaining the maximum product advantage from these processes has been the requirement to leave a rather high safety margin to avoid cutting a slit through the paper during each process. Such cut-through, of course, tends to allow adhesive to seep through the backing sheet and stick to adjacent sheets, thereby completely obviating the advantage sought by using a non-perforating process to selectively weaken the paper.
While it is undesirable to have such adhesive-penetrable weak-spots in the backing sheet, it is equally undesirable to have tear lines which are supposed to crack open when deliberately flexed by the user but which do not do so because the line is not weak enough. Therefore, it will be understood that a selectively weakened line must be deep enough to allow easy splitting but also must be strong enough so that it will not split when it is not supposed to split (for example, by tearing at split points not subjected to deliberate flexing. Such secondary split lines are often provided on release sheets of the so-called "universal-split" type and which are particularly described in U.S. Pat. No. 3,006,793 to Wheeler.)
In many prior art backing sheets, the extra lines were so weak that the sheet would tear at the extra lines and greatly slow down the stripping of the backing sheet and consequently, delay the label-applying process. The compressed paper lines of the instant invention have a relatively high residual strength when compared to the low resistance to breakage when flexed along a score line. As has been indicated above, this strength is a consequence of the uniformity with which weakening may be achieved by use of the present method and apparatus.